It is known to use a multiplier for gain control. The control signal is applied to one input of the multiplier and the input signal is applied to the second input of the multiplier. The output signal is present at the output.
In the case of known gain control, the output signal is linear with respect to the control signal. Given an input signal of 1 volt and a control signal between 0 and 10 volts, it is possible to realize linear gain properties having factors between 0 and 100. That is to say that a gain factor of 100 is produced for a control signal of 10 volts, and for a control signal of 5 volts the gain factor is 50. If the gain factors are expressed in dB, the gain factor of 100 corresponds to a value of 40 dB and the gain factor of 50 corresponds to the value 34 dB. A linear amplifier is therefore unsuitable for dB linearity, since the changes in the output signal have a great effect in the lower control ranges whereas, in the upper control ranges, the changes in the controlled variable do not have much of an effect on the output signal.
The gain factors lie between the values 1 and 1000 in the IF range. These gain factors must be distributed uniformly over the control voltage, and dB linearity between the output voltage and the control signal is therefore to be aimed at.
It is furthermore known from U.S. Pat. No. 5,057,717 and U.S. Pat. No. 4,004,141 to change an output voltage as a logarithmic function of the input voltage.